A Circular Polarization Selective Surface (CPSS) is a finite-thickness surface that predominately reflects one sense, or handedness, of a circular polarization (CP) of an incident electro-magnetic (EM) wave, and predominantly transmits an EM wave of the other sense of CP. An ideal reciprocal CPSS acts either as a mirror or a transparent window, depending on the sense of CP of the incident wave. A reciprocal CPSS is one for which the sense of CP of the predominantly reflected wave is the same as that of the incident wave. This is opposite to an ordinary reflection from an interface between two dielectric media or from a common metallic mirror, wherein the sense of the predominant CP of the reflected wave is opposite to that of the incident wave. Furthermore, the general operation of a reciprocal CPSS typically remains the same regardless of whether the CPSS is illuminated from one side or the other. In its simplest form, a prior art CPSS is a two-Dimensional (2D) periodic array of identical CPSS elements that lacks longitudinal reflection symmetry, is reciprocal, and with a Cartesian tiling configuration. In the context of this specification, the longitudinal direction is the direction that is normal to the CPSS and is the preferred direction of propagation of the incident wave. A CPSS is typically designed to CP-selectively reflect or transmit incident EM radiation of a particular frequency f, which is referred to hereinafter as the operating frequency, or simply the frequency.
U.S. Pat. No. 3,500,420 issued to Pierrot discloses an example of a CPSS reflector, wherein the main element is a crankwire that is illustrated in FIG. 1. Here, a crankwire is a conductive wire that is bent to be comprised of 3 mutually perpendicular conducting segments. In the Pierrot design, the lengths of two perpendicular end segments, which are also referred to herein as transverse segments, is 3λ/8, while the length of the middle, or longitudinal, segment is λ/4, with the total length of the crankwire equal to one wavelength λ. The relative orientation of the two transverse segments, i.e. the handedness of the geometry, dictates the operation of the CPSS element as to which sense of CP will be reflected upon being illuminated with a CP plane wave incident in the normal direction, i.e. a direction parallel with the longitudinal segment. Using Cartesian notation, when the longitudinal segment is aligned with the Z direction as illustrated in FIG. 1, the bottom transverse segment is aligned with the +X direction and the top transverse segment is aligned with the +Y direction, the crankwire reflects Left-Hand Circular Polarization (LHCP) when illuminated from the top or bottom, and a corresponding CPSS is referred to as a LHCPSS. With the top transverse segment aligned with the +X direction and the bottom transverse segment aligned with the +Y direction, the crankwire reflects Right-Hand Circular Polarization (RHCP) when illuminated from the top or bottom, and a corresponding CPSS is referred to as a RHCPSS. The crankwire has the same general operation whether it is illuminated from one end of its longitudinal axis or the other.
The operation of Pierrot's crankwire under normal incidence is as follows. Because the two transverse segments are orthogonal to one another, the EM coupling between them is negligible. Hence, one transverse segment does not create EM blockage for the other transverse segment as the incident wave propagates at normal incidence through the cell. Due to the λ/4 separation between the two perpendicular transverse segments, a normally incident plane wave of one sense of CP would induce two in-phase currents on the two transverse end-segments whereas a normally incident plane wave of the other sense of CP would induce two out-of-phase currents.
The two in-phase currents cooperate to produce a strong scattering response whereas the two out-of-phase currents nearly cancel one another to produce a weak scattering response. With the in-phase condition, the one-wavelength crankwire becomes resonant so that the current distribution over the entire length of the wire is sinusoidal-like, with a peak on each transverse segment and a null at the mid-point of the longitudinal segment. The relative orientation of the transverse segments that determines the handedness of the crankwire, and the λ/4 spacing between the transverse segments ensure that the sense of CP of the reflected wave is the same as that of the incident wave, as explained in more detail below. Hence, the reflected wave is strong and the sense of its CP is the same as that of the incident wave. In contrast, the total transmitted field is very weak because the transmitted scattered wave is equal and opposite to the incident wave, and because the total transmitted field is the vectorial summation of the incident wave and the scattered wave. With the out-of-phase condition, the two out-of-phase currents produce a bell shape current distribution with a small peak value at the mid-point of the longitudinal segment. Since this produces only a very weak scattering response, the incident wave goes through the crankwire with little or no disturbance as if the crankwire were absent.
In more specific terms, the operation of Pierrot's crankwire in FIG. 1 under normal incidence is as follows. The LHCP incident plane wave can be decomposed in two linearly polarized (LP) plane waves that propagate in the same direction (say, the −Z direction), with the two linear polarizations mutually orthogonal in space (one LP plane wave polarized with its E field along, say, the +X axis, and the other LP plane wave polarized with its E field along, say, the +Y axis) and phase shifted −90 degrees in time (the E field component along the +X axis varies as cos [ω(t−t0)] while the E field component along the +Y direction varies as sin [ω(t−t0)] wherein t0 is an arbitrary time origin). The temporal period of the signal of radian frequency ω=2πf is T such that (ωT/4)=90 degrees. At time t=t0, the X-polarized wave at the top face of the cell has maximum value because cos [ω(t−t0)]=1 but it propagates through the Y-parallel segment without interaction because it is orthogonal to the Y-parallel segment, and reaches at time t=t0+(T/4) the bottom face of the cell where it induces a =X-directed current on the X-parallel segment. At time t=t0, the Y-polarized wave at the top face of the cell has zero value because sin [ω(t−t0)]=0 and induces no current on the Y-parallel segment. At time t=t0+(T/4), the Y-polarized wave at the top face of the cell has maximum value because sin [ω(t−t0)]=1 and induces a negative Y-directed current on the Y-parallel segment. Hence, at t=t0+(T/4), the induced current is most intense on both transverse segments and corresponds to the sinusoidal current distribution of a 1λ long wire under resonance. Hence, when the induced current on the X-directed segment has the negative polarity given by the −X direction, that on the Y-directed segment has the negative polarity given by the −Y direction. Thus, both induced currents are in-phase as one current pushes while the other one pulls on charges. In contrast, when the incident plane wave is polarized RHCP, the E field component along the +X axis varies as cos [ω(t−t0)] while the E field component along the +Y direction varies as −sin [ω(t−t0)]. Again, at time t=t0, the X-polarized wave at the top face of the cell has maximum value because cos [ω(t−t0)]=1 but it propagates through the Y-parallel segment without interaction because it is orthogonal to the Y-parallel segment, and reaches at time t=t0+(T/4) the bottom face of the cell where it induces a −X-directed current on the X-parallel segment. At time t=t0, the Y-polarized wave at the top face of the cell has zero value because −sin [ω(t−t0)]=0 and induces no current on the Y-parallel segment. At time t=t0+(T/4), the Y-polarized wave at the top face of the cell has maximum negative value because −sin [ω(t−t1)]=−1 and induces a +Y-directed current on the Y-parallel segment. Hence, at t=t0+(T/4), the induced current on the X-directed segment has the negative polarity given by the −X direction while that on the Y-directed segment has the positive polarity given by the +Y direction. Thus, both induced currents are out-of-phase as both currents attempt to push or pull on the charges simultaneously. In practice, the residual current is not exactly zero and its distribution along the wire has a bell-like shape to it.
Different variations of the Pierrot design have been disclosed, including ones using printed circuit boards with metalized via holes to implement the crankwires. One variation of Pierrot design is disclosed in an article by I-Young Tarn and Shyh-Jong Chung, “A New Advance in Circular Polarization Selective Surface—A Three Layered CPSS Without Vertical Conductive Segments”, IEEE Transactions on Antennas and Propagation, Vol. 55, No. 2, February 2007, pp. 460-467. It involves using the Printed Circuit Board (PCB) technology to implement the crankwires, with the metallized via-holes that realizes the longitudinal segments of the crankwires being replaced by conducting traces on intermediate layers between the top and bottom surfaces of the PCB. Due to the partial vertical alignment of one strip with the strip on the next layer, the EM energy flows vertically from one strip to the other by capacitive coupling. This permits to electrically connect the two transverse segments of the crankwire without using a continuous conductor between them.
One drawback of CPSS of the Pierrot type composed of a periodic array of the crankwires of the same handedness is that its performance is satisfactory only at or near normal incidence, and quickly degrades with oblique incidence.
This issue is addressed by U.S. Pat. No. 5,053,785 to Tilston et al, which is incorporated herein by reference and which discloses a CPSS element 20 in the form of a dipole arrangement that is illustrated in FIG. 2, and which has a 2-fold rotational symmetry. The CPSS element 20 of Tilston includes two perpendicular half-wavelength dipoles 22 and 24 separated physically by a λ/4 spacing but connected electrically by a λ/2 transmission line 30. The operation of Tilston's design is as follows. Due to the λ/4 separation between the two perpendicular transverse dipoles 22, 24, a normally incident plane wave would induce currents on the two perpendicular dipoles 22, 24 such that the two voltage travelling waves present at the two opposite ends of the transmission line would be equal in magnitude but in-phase for one sense of CP, and out-of-phase for the other sense of CP of the incident wave. The induced currents are equal in magnitude because the EM coupling between the two perpendicular dipoles is very weak, owing to the dipoles being mutually perpendicular. Hence, one dipole does not create EM blockage for the other dipole as the incident wave propagates through the cell. From the longitudinal symmetry of the transmission line 30, the two equal-magnitude in-phase voltage travelling waves at the two opposite ends of the transmission line produce a virtual open-circuit at the mid-point of the transmission line whereas the two equal-magnitude out-of-phase voltage travelling waves produce a virtual short-circuit at the mid-point. Since the transmission line is electrically a half-wavelength long, a virtual short-circuit at the mid-point of the transmission line is transformed through a λ/4 transmission line into an open-circuit at the port of each perpendicular dipole connected at each end of the transmission line, and conversely, a virtual open-circuit at the mid-point is transformed into a short-circuit. The orthogonal half-wavelength dipoles produce a strong scattering response when their terminals are short-circuited because each dipole acquires a resonance length of a half-wavelength. In contrast, the two orthogonal half-wavelength dipoles produce a weak scattering response when their terminals are open-circuited because each dipole is segmented into two non-resonant λ/4 wires. The sense of the CP that is reflected for Tilston's design depends on the connection of the longitudinal transmission line to the two dipoles at its two ends. In fact, this connection is the same as if Tilston's design were two “back-to-back” crankwires. Hence, the explanation for the sense of the CP being scattered for Tilston's design is the same as that which was given for Pierrot's crankwire since the fact that the lengths of the transverse segments are different between Pierrot's crankwire and Tilston's dipoles does not affect the sense of CP being scattered. One advantage of the Tilston's design is that is has a 2-fold rotational symmetry, which has been shown to provide a good performance under oblique incidence. Notably, U.S. Pat. No. 5,053,785 is silent as to possible solutions to a problem of incorporating the half-wavelength transmission line in the quarter-wavelength spacing that corresponds to the thickness of the cell, and further is silent on possible performance of the suggested design. Furthermore, the half-wavelength dipoles need to be rotated 45 degrees to lie on the diagonals of the cells in order to fit within cells that are no larger than a half-wavelength in order to avoid the formation of grating lobes and the presence of higher-order modes of propagation.
FIG. 3 illustrates another prior art CPSS that may be referred to as a CP-LP-CP cascade design, which is disclosed by U.S. Pat. No. 3,271,771 to P. W. Hannan et al. It includes a cascade of two circular polarizers of opposite handedness sandwiching a linear wire-grid polarizer. Its operation involves converting the input CP into a Linear Polarization (LP), filtering the LP with a wire grid and reconverting the output LP into CP. The CPSS operation would be changed from reflecting one sense of CP to reflecting the other sense of CP by rotating the wire grid by 90 degrees. One disadvantage of the cascade design is that its performance under oblique incidence is limited because the linear polarization filter works best only under normal incident EM illumination. Also, the realization of the CP-LP-CP cascade design is much thicker than those of Pierrot's or Tilston's designs, which is a disadvantage in terms of volume, weight and space.
An object of the present invention is to provide an improved CPSS which addresses at least some of the disadvantages of the prior art, and which provides improved performance in at least some applications.